(a) Field of the Invention
Generally, the present invention relates to a fuel cell. More particularly, the present invention relates to a separator for a fuel cell using a graphite foil, a manufacturing method thereof, and a fuel cell stack including such a separator.
(b) Description of the Related Art
As is well known in the art, a fuel cell produces electric power by an oxidation reaction at an anode and a reduction reaction at a cathode. The anode and the cathode are formed with a catalyst layer having platinum or platinum-ruthenium metal for accelerating the oxidation and reduction reactions.
At the anode, fuel gas (for example, hydrogen) is supplied thereto and is divided into ions (for example, protons) and electrons through the oxidation reaction. At the cathode, the divided ion bonds with a reduction gas (for example, oxygen) to form water. Final products of such reactions are electricity (i.e., electron movement from the anode to the cathode), water (i.e., a chemical bond of hydrogen and oxygen), and heat. A fuel cell stack is usually provided with a cooling device for removing such heat.
The water formed at the cathode is usually formed as vapor or liquid, and such water is removed by a strong stream of reduction gas (oxygen or air) flowing at a cathode side.
FIG. 1 is a schematic sectional view of an exemplary fuel cell stack according to the prior art.
Usually, a fuel cell stack is formed by stacking a plurality of unit cells 100.
Such a unit cell 100 includes a proton exchange membrane 110 (for example, a polymer electrolyte membrane). An anode 121 and a cathode 122 are formed at both sides of the proton exchange membrane 110. The proton exchange membrane 110 and electrodes 121 and 122 form a membrane electrode assembly (MEA) 130 by hot pressing. Fluid diffusion layers 125 are formed to the exterior of the electrodes 121 and 122 of the MEA 130.
MEAs 130 of adjacent unit cells are separated and supported by a separator 150. The separator 150 is formed with a flow field 151 for supplying fuel gas (e.g., hydrogen, or methanol in the case of a direct methanol fuel cell) to the anode. In addition, the separator 150 is also formed with a flow field 152 for supplying oxygen or air as a reduction gas to the cathode, and also for exhausting water. A gasket 160 is applied between the separator 150 and the MEA 130, for preventing leakage of gas/liquid flowing through the flow fields 151 and 152.
The unit cells 100 including the MEA 130, the separator 150, and the gasket 160 are stacked in series to form a high voltage. The stacked unit cells are conjoined by, e.g., current collectors and end plates 170 disposed at ends thereof.
As can be understood from the above description, a separator in a fuel cell distributes reaction gases (i.e., fuel gas and reduction gas) through the fuel cell stack, separates a fuel gas (e.g., hydrogen or methanol) and a reduction gas (e.g., oxygen or air), and electrically connects adjacent unit cells by providing an electron passage between an anode and a cathode of adjacent unit cells. In addition, the separator has a heat exhaust structure for exhausting heat produced by the oxidation-reduction reaction of the fuel cell stack, and provides mechanical strength for supporting the stacked unit cells.
In order to accelerate movement of hydrogen ions (i.e., protons) produced at the anode to the cathode through a polymer electrolyte membrane, the polymer electrolyte membrane should be hydrated to contain an appropriate amount of moisture. The hydrated polymer electrolyte membrane prevents movement of electrons therethrough while allowing movement of hydrogen ions.
When the polymer electrolyte membrane is not sufficiently hydrated, ion conductivity of the polymer electrolyte membrane is lowered, and therefore performance of a fuel cell is deteriorated. To the contrary, when the polymer electrolyte membrane is excessively hydrated, small pores forming a triple-phase boundary of reaction are blocked (which is usually called flooding), and thereby the reaction area of the electrodes reduces, resulting in deterioration in performance of the fuel cell.
Therefore, in the case that the water formed at cathodes is not promptly exhausted, reaction gas is not sufficiently supplied to the catalyst layer, and therefore performance of a fuel cell is deteriorated.
Many separators, including an exemplary one disclosed by U.S. Pat. No. 4,988,583, have serpentine flow fields for fuel and reduction gases. This is mainly for utilizing a pressure drop along the flow fields for efficient exhaust of water formed at the cathodes.
The water formed at the cathodes is in the form of vapor, near the entry of reduction gas flow-field channel. However, as it flows through the reduction gas flow-field channel, it becomes of two phased, as mixed liquid and vapor. In this case, liquefied water drops fill the pores of the cathodes, and accordingly, the effective active areas of the catalyst layers become reduced. In addition, liquefied water requires a high pressure of reduction gas for exhaust thereof.
Therefore, energy loss occurs by a pressure drop of reduction gas between entry and exit of flow fields, and reaction gas is much consumed for stable realization of the reduction reaction at a high flow speed. Therefore, if water exhaust of a separator having serpentine flow fields becomes more stable and efficient, it will promise a reduction of energy loss by the pressure drop of reduction gas between the entry and exit of the flow fields and reduction of consumption of reaction gas.
Graphite or carbon composite materials are widely used for a separator for a polymer electrolyte membrane fuel cell. The graphite and the carbon composite material show strong anti-corrosiveness to the oxidation-reduction reaction of a fuel cell, and also have a merit of low bulk density in comparison with metallic materials (e.g., stainless steel).
When the graphite or the carbon composite material is used as a material for a separator, according to the prior art, a resin such as thermosetting or thermoplastic resin is usually added to the separator material in order to prevent movement of hydrogen by filling micropores of the separator, and also for easy forming during the molding process. However, the resin included in the separator causes an increase of volume resistance with respect to movement of electrons, and thereby deteriorates performance of a fuel cell. Furthermore, the resin increases contact resistance between cells.
As an exemplary method for reducing an increase of contact resistance between cells cause by the resin in the separator, European Patent Publication No. EP1253657A1 discloses a method in which rib surfaces of flow fields of a separator are etched in an alkaline solution such that the resin in the surface area of the rib may be removed.
According to the prior art, the manufacturing process for a stable and useful separator using a graphite or a carbon composite material has been very complex, non-productive, and non-efficient. Therefore, if a separator using graphite or a carbon composite material can result in higher performance and be appropriate for mass production, it promises a substantial decrease in production cost of a separator and in turn production cost of a fuel cell, as well as enhancement of performance of a separator.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known in this country to a person of ordinary skill in the art.